SALMON, Idaho (Reuters) - Firefighters on Saturday gained control of a fierce brush fire that has raged for three days on the sprawling grounds of the government's leading nuclear research lab in the high desert of eastern Idaho.

By midday, fire crews at the Idaho National Laboratory had carved containment lines around 90 percent of the blaze, which has scorched about 50,000 acres of parched sagebrush and grasslands within the 890-square-mile complex.

"Right now, it's looking pretty good," lab spokeswoman Carisa Schultz said about the likelihood of the fire being extinguished by day's end.

The blaze, ignited on Thursday by a lightning strike, had prompted the evacuation of nearly 100 workers that evening from a facility used for processing spent nuclear fuel and radioactive waste. That installation lies about 35 miles west of Idaho Falls, a city of roughly 57,000 people.

But personnel were allowed to return and lab operations as a whole resumed as normal the next day, lab officials said.

A 19-mile stretch of a state highway running through lab-owned lands west of the rural community of Mud Lake was reopened on Friday night.

Lab officials said one firefighter suffered heat exhaustion but no other injuries were reported.

No radiologically contaminated areas were breached by flames, and no airborne contamination has been detected during the fire by routine monitoring from either the state of Idaho or the U.S. Energy Department.

The only property damage noted was to some power poles on the complex, but electricity supplies to the facilities were not disrupted, lab emergency director Steve Dunn said.

"Risks to radiological facilities and important buildings at INL are manageable because of natural and constructed firebreaks, the predominant use of noncombustible construction materials and the presence of reliable water supplies and automatic fire suppression systems at the site," the lab said in a statement.

A separate, much smaller fire erupted on the northwest edge of the site earlier in the week, touched off by sparks from the bare rim of a blown tire dragging on pavement, but that blaze was confined to 100 acres before it was snuffed out.

Some 6,000 employees and contractors work at the Idaho National Laboratory, which houses three working reactors and is considered the Energy Department's top facility for nuclear research and development.

The blaze ranked as the largest of several that have charred tens of thousands of acres across Idaho and the Northern Rockies in recent days, including parts of Montana, Yellowstone National Park and northwestern Wyoming.

(Writing by Steve Gorman; Editing by Tim Gaynor)

mochila insert follows